1. Field of the Invention
The present invention relates to an arrester for a gas insulated switchgear device and, in particular, relates to an arrester for a gas insulated switchgear device in which a plurality of zinc oxide resistance elements are piled up to constitute a columnar piled body and a plurality of equalizing rings are disposed around the columnar piled body in a discrete manner and in the piling direction.
2. Description of Related Art
An arrester for a gas insulated switchgear device is constituted by disposing a columnar piled body formed by piling up a plurality of zinc oxide resistance elements in a metallic closed container filled with an electrically insulating gas. In such structure, it is generally known that a division rate of an applied voltage to the respective zinc oxide resistance elements forming the columnar piled body along the piling direction is non-uniform due to stray capacitances between the columnar piled body and the closed contaiiner.
A conventional arrester for a gas insulated switchgear device which improves the above problem is disclosed in JP-B-64-1913, which corresponds to U.S. Pat. No. 4,340,924 and is illustrated in FIG. 9. Namely, a metallic closed container 2 filled with an electrically insulating gas is connected to a non-illustrated gas insulated switchgear device via an insulating spacer 3, and in the closed container 2 a columnar piled body 1 is disposed, the lower end of which is grounded and the upper end of which is connected to a main circuit conductor. The columnar piled body 1 is formed by piling up a plurality of zinc oxide resistance elements in the axial direction of the closed container 2.
At the side of the main circuit conductor 4 on the columnar piled body 1, an umbrella shaped shield member 26 is provided. At the lower end of the umbrella shaped shield member 26, a potential equalizing ring 20, such as a shield ring surrounding the outer circumference of the columnar piled body 1, is fixed. Further, a plurality of potential equalizing rings 21 and 22, arranged in the pile axis direction of the zinc oxide resistance elements with a predetermined spacing distance and similarly surrounding the outer circumference of the columnar piled body 1, are disposed, and these potential equalizing rings 20, 21 and 22 are electrically and mechanically connected to the side of the main circuit conductor 4 via conductors 30 and 31. The distances between the respective potential equalizing rings 20, 21 and 22 are determined in such a manner that equipotential lines passing between the respective potential equalizing rings 20, 21 and 22 are to be connected to the corresponding equipotential planes on the columnar piled body 1 as illustrated by dotted lines, whereby the division rate of applied voltage on the respective zinc oxide resistance elements of the columnar piled body 1 is made uniform throughout the entire piling up direction.
With the modified conventional arrester for a gas insulated switchgear device, through the above mentioned provision of the respective potential equalizing rings 20, 21 and 22, the division rate of the applied voltage on the respective zinc oxide resistance elements of the columnar piled body 1 is remarkably improved throughout the entire piling up direction. However, according to an analysis by the present inventors, when the length of the columnar piled body 1 increases in comparison with the diameter of the closed container 2, it was found out that among the respective potential equalizing rings 20, 21 and 22, the electric field most concentrates at the lower side of the lowermost potential equalizing ring 22, namely the ring located near the grounding potential side, and a division rate of an applied voltage on the zinc oxide resistance element of the columnar piled body 1 facing the lowermost potential equalizing ring 22 located near the grounding potential side becomes higher than those of the zinc oxide resistance elements in the other locations.